One common type of electrohydraulic servolvalve comprises a torque motor which receives an electrical signal and positions a flapper between a pair of opposed nozzles to control a spool valve and a feedback spring connected to the flapper and to the spool of the spool valve.
Such servovalves are normally configured to contain a pilot stage and a power stage. The pilot stage is the portion of the valve which converts an electrical signal to mechanical motion and the power stage is the portion which amplifies the pilot stage power to a practical level. The pilot stage is a sensitive and precisely manufactured device. The device contains four air gaps commonly called the upper pole gaps and the lower pole gaps. It is very important that these gaps are manufactured to be equal to each other as well as to a specific size for the particular torque motor under construction. A typical size for the gap is 0.010 to 0.015 inches with all four gaps ideally within 0.0005 inches of each other. Although it is slightly less than ideal, it is acceptable to have the lower gaps equal to each other and the upper gaps also equal to each other, but the lower gaps may be slightly (0.001 inches) different than the upper gaps. There are a large number of parts that ultimately determine the gap dimension. It is thus not practical to hold the critical dimensions close enough to provide the necessary gap control.
One solution to the problem is to grind the total air gap to a specific dimension for the magnet and pole piece subassembly. The armature ends are ground to a dimension equal to the total pole gap minus two times the desired air gap. The torque motor is assembled and the resulting air gaps are observed. Shims are then replaced with other shims which will bring the air gaps to the desired uniformity by shifting the entire pole piece/magnet subassembly.
A second solution to the problem which has been proposed is to completely assemble the torque motor with parts such that all the gaps are smaller than desired, but not zero, and EDM processing all the gaps at one time. This process still requires fairly close tolerance control of many parts and may also require some shim adjustment for minor correction.
Among the objectives of the present invention are to provide a method of assembly which obviates the problems in the prior art.
In accordance with the invention, the armature is assembled on the spring tube/flapper subassembly of the torque motor and the joint between the armature and the flapper subassembly comprises a one part, heat cured thermosetting adhesive.
More specifically, the method of obtaining torque motor air gap symmetry is provided which comprises forming the pole piece/magnets subassembly to provide a desired total pole air gap on both ends of the armature, forming the armature ends so that they are equal to the total pole gap minus twice the desired nominal air gap, assembling the torque motor before attaching the armature to the spring tube/flapper subassembly, positioning the armature on the spring tube/flapper subassembly and the pole piece/magnet subassembly in a relative position to one another by providing spacers between the lower edges of the armature and the gap of the pole pieces, providing wedges between the upper surfaces of the ends of the armatures and the gap of the upper pole piece, providing a joint between the tube and the armature by a hardenable material to bond the armature and the spring tube subassembly, and permitting the joint to harden and set.